The present invention generally relates to Electrostatic Discharge (ESD) testing, and more specifically relates to a design methodology for preventing functional failure caused by ESD.
Semiconductor products are becoming increasingly complex, with more and more functions being integrated into each component. Integrated Circuits (IC's), referred to as Systems on Chip (SoC) or Systems in Package (SiP), are reaching new levels of integration—various types of circuit blocks are combined inside a single package on a single die or combinations of multiple dice. These include analog, digital, radio frequency, opto-electronic, and microelectronic devices. Also, the process technologies are scaling according to Moore's Law, with devices operating in very low voltage regions of one volt and less. This advancing level of circuit complexity is pushing technology to a new echelon, which affects the ESD immunity of semiconductor IC's.
Research shows that up to 25% of field returns are due to ESD damage, which results in huge costs to semiconductor manufacturers in terms of materials, recycling, and protecting against ESD in manufacturing areas. Additionally, there could be adverse effects on a vendor's reputation and repeat sales to end users. In turn, ESD protection can cost manufacturers up to 10 percent of their annual revenue in terms of protection, designing prevention methods, and processing field failures.
ESD testing standards include models such as Machine Model (MM) and Charged Device Model (CDM). The elements that define these tests include a real-world example of a charged capacitance and a discharge path that occurs.
There exists a set of standards that define how ESD tests should be conducted for qualification testing. These standards, set forth by JEDEC and the ESD Association, form the basis of the ESD testing sequences performed. For example, a vendor will submit several fully functional IC's to an ESD testing service. The tester will be calibrated and verified using an oscilloscope, and the high voltage discharge waveforms will be verified before testing begins. The IC's are then installed, and ESD testing commences. The first sequence is to sweep current and measure voltage, to develop a series of curve traces for each of the pins on the IC. When the voltage application sequencing beings, various curve traces are taken throughout the process in order to identify worst-case failing conditions. If the curve-trace shifts after the ESD pulse is applied, it is assumed that a failure has occurred. Comparisons are typically made throughout the testing process to identify shifts or shorts that may have developed as a result of the ESD pulses. After testing is complete, the IC's are sent to the automated test floor to have final parametric and functional testing performed (ATE testing), which is the final verification of a passing or failing test for the IC.
With regard to the different testing methods that are used, Machine Model (MM) was developed in the 1990's as a way of modeling what happens in auto manufacturing facilities when a machine becomes electrostatically charged, and discharges into an IC when it comes in contact with it. This model is used less today than it was in the past.
The Charged Device Model (CDM) test method is steadily taking the place of the MM testing method for today's ESD testing. This method simulates what happens in an automated manufacturing environment when an IC becomes triboelectrically charged, and the part discharges when it comes in contact with a grounded conductor. The CDM discharge is a fast transient, which takes place over a couple of nanoseconds at most. This is a more challenging device to model, and is based on the size and the capacitance of the IC being tested, with little impedance to reduce the current. This can result in high currents (i.e., well above five to six amperes) being discharged in a short duration of one nanosecond or less. As such, the CDM test method stands as the most practical real world test for ESD issues today.
The CDM testing procedure consists of the device under test (DUT) being placed on the tester with the leads or contacts facing up. Specifically, the DUT is placed on a thin layer of flame retardant (FR-4) material, and under the flame retardant material is a metal field plate. The flame retardant material which acts as an insulating capacitor between the DUT and the metal field plate. A high voltage power supply is used to raise the metal field plate to the required CDM test voltage level (typically between 500 and 1000 volts), and a robotic pogo probe is brought into contact with a pin under test. A high-current electric discharge takes place, which is verified by monitoring the ground connection of the pin under test. Subsequently, the metal field plate is grounded to remove any residual charge and the process is repeated. CDM industry standards require that each pin on the DUT receives three positive and three negative pulses, resulting in six total discharges per pin. A device is deemed to have failed if, after exposure to ESD, the device no longer meets its data sheet specifications.
CDM failures typically occur in two areas on the chip: in the core as dielectric failures due to voltage build-up during the CDM discharge, or in the Input/Output circuitry as metallization or junction failures due to current crowding.